CHAPTER XXXI 

 MATERIALS 



Importance of Quality. The durability of a machine 

 depends largely upon the quality and character of the mate- 

 rials used in the construction of it. It is obvious that a 

 knowledge of the properties of these materials wall be use- 

 ful to those who have to do with the selection and manage- 

 ment of machinery. 



Wood. Twenty-five to forty years ago the framework 

 of farm machinery was made largely of wood. At that 

 time wood stock of the first quality and of the most desir- 

 able varieties could be obtained cheaply. The increase in 

 the cost of wood, due to its scarcity, and the decreasing cost 

 of manufacturing iron and steel has led to a more extended 

 use of metal. The wood used in the construction of farm 

 machinery, since it must undergo rather severe service, 

 should be of selected quality. Carefully selected, well-sea- 

 soned heartwood in the only practical kind to use. 



Wood is influenced more or less by moisture, and for 

 that reason should be carefully protected by paint. A 

 combination of iron and wood parts is likely to give trouble 

 by becoming loose, due to the shrinking of the wood. Parts 

 subject to much vibration, like the pitman of a mower, can 

 best be made of wood. Excessive vibration and shocks 

 tend to cause steel to crystallize. 



Some of the more common varieties of woods and forms 

 of metal used in the construction of farm machinery will 

 now be discussed. 



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